Diffusor-Absorber Design

If you look at the studio pics my link you will see the same diffusor, except I did not use peg board, I used a thing solid board. They work great. I used sound deadening polyester inside mine.

Alan.
 
What do you guys think?
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First off, these are simply Polys, with some HF absorption via the holes/batting. Poly's do NOT diffuse...they only scatter sound in different directions.

Lets define a diffuse sound field. According to Alton Everest, a diffuse sound field is one where sound decays equally at all frequencies. This typically has nothing to do with Polys, of which simply scatter sound in a hemispheric pattern, although, only down to frequencies with 1/4 wavelengths equal to the physical size of the Poly. Real diffusers scatter sound in relationship to TIME. Which DOES affect the decay of Mid/High frequencies. The only way you can affect the decay of LF's is through ABSORPTION...at least in a small room. Typically, DECAY is defined by the RT-60 of the room. Which means, at any given frequency..the time it takes for the sound to decay 60db. However, for reasons too difficult to explain here, small rooms really do not really have an RT-60. .

One thing to consider though, is the physical size of the room in comparison to the poly's. Rooms with width/length dimensions smaller than 10' will perform better if LF absorption is the main treatment. Poly's perform better in larger rooms.

At the end of the day, the REAL problem with small rooms is Low Frequencies. You can't diffuse any frequency with wavelengths longer than the physical dimensions of the room itself. The BEST thing you can do in a typical small room is ABSORB low frequencies, and the easiest/best way to achieve this is by using SUPERCHUNKS of rigid fiberglass in all corners of the room, because low frequencies TERMINATE in the corners. Or at minimum, diagonal panels at least 4" thick placed across the corners, floor to ceiling, with batt type insulation behind. Place a scrim of 4mil plastic sheeting over the face for HF reflection too.

Well, that's my NON expert .02. I like poly's, but not in rooms smaller than 10' on the longest dimension.
 
I was thinking of building a larger one for use behind my drumkit. One question though, how do you keep the filler from falling out the bottom? I understand the fabric is wrapped tight but would it not sag and show a bulge at the bottom?
 
but not in rooms smaller than 10' on the longest dimension.
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Thanks for the awesome info Fitz! I learned something new :) I have a question for you though. My room is approx 21' 8" x 11' 7". Would I benefit from any poly's, or should I just continue to fill my room with trapping?

Thanks, Drew
 
Rick is correct. But for your 21 by 11ft. space, you can definitely use some polys. :)

I recommend placing them in corners over corner traps so you get the trapping AND dispersion. This helps to retain some HF in the room. A good rule to follow in tracking rooms is; Never let untreated surfaces face each other. And unlike control rooms, asymmetry is desired in tracking rooms.

If you want more diffusion from your poly, you can do like the photo 004a.jpg - otherwise, I highly recommend using a PRD for walls that are close to the kit or stick to absorption.

Cheers,
John
 
Hey John! Thanks for chiming in. I saw those same type corners on your design for the "outback" studio. :D But I have a question for you. To what extent do Slots/Slats actually "diffuse"?

And in corners, what advantage do they have vs simple "superchunks" and a thin plastic scrim fro HF reflection? Of course, your geometric "poly" shape of the slats would seem to act to "scatter" sounds, but I still don't understand the relationship of the slat/slot widths, and slot depths. Are you calculating something or is this kind of arbitrary. I know Mr. Sayers and others use random units of slats/slots, but these are all the same.

And now that I think about it, if memory serves me, there was a time when even Eric Desart was questioning the performance of slat/slots over a sealed enclosure. Although, and please correct me if I'm wrong, but I think this technique was actually implemented clear back in the 40's/50's by an acoustician at Captiol records. So they must work. But this is why I get so ...well, lets just say ..bewildered..by studio acoustics. So many differences of opinion. WHO DO I TRUST? I mean..frankly, I've had an acoustician in Europe actually call me to tell me he agreed with many of the points I ranted about in this regard, in a thread over at slutz. THAT tells me something.:rolleyes::D In fact, he had a LOT to say about this biz. And it wasn't very nice either. But enough of that.:confused:

Anyway, thanks again for chiming in. I've grown kinda gun shy sticking my neck out about this stuff lately. It helps to have a "pro" confirm what my belly tells me.:p
 
Rick,

Thank you.

Actually, the rating system for diffusion is similar to rating absorption - from 0.0 to 1.0. - And polys are indeed diffusers of sound. Ideally you should be approximately an arc's distance from them before a decent diffusive field develops.
To quote from Acoustic Absorbers and Diffusors by Cox & D'Antonnio;
"Diffusion coefficient, (d), is a measure of the uniformity of the reflected sound.The purpose of this coefficient is to enable the design of diffusers, and to also allow acousticians to compare the performance of surfaces for room design and performance specifications.

Scattering coefficient, (s), is a ratio of sound energy scattered in a non-specular manner to the total reflected sound energy. The purpose of this coefficient is to characterize surface scattering for use in geometrical room modelling programs."

That said... The random slat widths used for scattering are simply that. Phillip Newell uses a 5 - 3- 4 sequence on his slat widths, but it really has no significance other than the percentage of absorption vs reflection/diffusion - which the slat sizes and gap sizes determine. As long as the slats are not covering a sealed box, they do not create a Helmholtz response and the LF acoustic wave does not even 'see' the slats & therefore continues into the trap where it is summarily 'dealt with'. ;)

Here is another quote from the same book concerning testing done on poly-cylindrical diffusors (emphasis added);
"A value close to zero has been measured for a concave surface designed to focus sound on a single receiver. A value of 1 can be measured for a small single semicylinder, but a single semicylinder on its own is not much use, because it cannot cover a wide area. As soon as more complex surfaces are introduced, such as a set of semicylinders, the diffusion coefficient is reduced, because of the lobing introduced. This lobing is unavoidable in extended structures, and so the diffusion coefficient is rarely close to 1 for usable and realistic diffusers and diffuser arrays. A single semicylinder may produce complete diffusion, but to cover a wall a set of semicylinders are needed. This is why it is important to measure application realistic samples, as the scattering from a single object is not representative of the response from a periodic or modulated array."

- Good stuff. :)

If anyone needs (or wants) further explanation, please ask.
Cheers,
John
 
Wow, that was fast. Isn't the Internet cool!!:D


- Good stuff.
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Yea, if I understood it.:confused::D

If anyone needs (or wants) further explanation, please ask.
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Ok, I'm asking.:p

In regards to this...
"A value close to zero has been measured for a concave surface designed to focus sound on a single receiver. A value of 1 can be measured for a small single semicylinder, but a single semicylinder on its own is not much use,
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:confused: Hmmmm, that "seems" to be a contridiction. If a small single semicylinder, say like the ones in your picture, equal a value of 1, but are "not much use"....that confuses me. Can you please elaborate or explain what I'm missing here. Slats or not, are not these units in the picture, "semicylinders"?


Furthermore, if they "arn't much use", what is the point? Why not use simple superchunks then?

Thanks for the previous explanation, but I need all the help I can get to understand this stuff.:)
 
Thanks John!
I recommend placing them in corners over corner traps so you get the trapping AND dispersion
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Would you recommend only placing them in corners, or would some on the walls be beneficial as well? If so, are all of my walls valid candidates, or just the walls with 21" of space between them?

I highly recommend using a PRD for walls that are close to the kit
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So walls that are close to my drum kit would benefit the most fro a PRD and not poly's. Or am I totally misunderstanding?

Thanks, Drew
 
Rick,

The semi-cylinders that were tested were 61 centimeters (24") across. The text was speaking of creating a diffuse field in the room, which would require a wall of polys. ;) (actually 4 walls hehe!) When/if you use several polys side-by-side you should vary their width and depth to partially avoid the lobing effect. We add slats to ours to increase high frequency & mid-range diffraction.
an example of the 'Normalized diffusion coefficient' for a 24" poly (radius 30cm or 12" - creating half a cylinder);
Frequency:100 /125 /160 /200 /250 /315 /400 /500 /630 /800 /1000 /1250 /1600 /2000 /2500 /3150 /4000 /5000
Coefficient:0.02 /0.00 /0.00 /0.00 /0.65 /0.96 /0.92 /0.96 /0.97 /0.90 /0.93 /0.95 /0.94 /0.95 /0.97 /0.98 /0.98 /0.98

I'd say that's pretty darn good for a 24" half-cylinder. - This exactly how posts and pillars help create a diffuse field in some cathedrals and music halls.
Now put up a 48" poly bent into an ellipse. The overall performance is slightly reduced for a broader and deeper coverage. I have found that from 2 to 4 times the radius of the poly is a good working distance. - Note; adding slats to polys improves diffraction/dispersion close to the unit so that instruments like drums can be within 1 radius distance without detrimental filtering effects. The diffraction lowers the intensity of the reflected energy. Also note, that specific units should be tested for their applicability to certain uses.
-- I hope this helps. :P

Drew,

By all means use polys on the walls, we have found them incredibly efficient - not to mention inexpensive to build. ;) If you do several polys side-by side, you should separate them by about 1/3 the width of each poly (varied) and vary the width and depth by at least 15%. This will help lessen the lobing effect. As I said before, the usable distance from polys are 1 to 4 radii. It is best to use slats on them if you are going to be at 1 radius. We typically make slats from 1 by 4s ripped down the middle - you end up with boards 3/4" thick and 1 5/8" wide. Space them apart from 3/4" to 1 1/4". Note; our slatted polys do not have a plywood skin, but are fabric covered absorbent traps that include a limp bag membrane.
So, slat traps or PRDs are good... polys are great too. It really depends on the overall design and what you are trying to accomplish. Usually for 'near' walls and ceiling, I would recommend the DIYer to place good thick absorption panels at those 'near' points to avoid specular reflections that cause destructive interference. - It also depends on where you place your microphones... and like Forest Gump said, "-- that's a hole 'nuther country..."

Cheers,
John
 
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